Circuits, such as integrated circuits (ICs), are commonly susceptible to damage by electrostatic discharge (ESD). For example, a functional component such as a metal-oxide-semiconductor field effect transistor (MOSFET) in an IC may have two functional elements that are separated by an epitaxial layer between the elements. When an ESD current enters one of these elements, the ESD current can damage the epitaxial layer such that the MOSFET becomes non-functional. Thus, an ESD protector may be implemented in a circuit to prevent an ESD current from entering the functional component.
FIG. 1 is a schematic diagram of a conventional IC 100 with functional circuitry 110 connected to an input or output terminal 120. A signal having one or more frequencies is applied at terminal 120 to engage functional circuitry 110. IC 100 may be connected to a power supply that includes a high-voltage power terminal and a low-voltage power terminal. In FIG. 1, the high-voltage terminal is shown as VDD and the low-voltage terminal is shown as electrical ground. In addition, a VDD-to-VSS ESD clamp 130 may be provided to direct an ESD current at the high-voltage or low-voltage terminal to the opposite terminal.
An ESD protector 140 is implemented in IC 100 to divert ESD current from terminal 120 of IC 100 away from functional circuitry 110 while allowing the signal applied at terminal 120 to pass to functional circuitry 110. By diverting the ESD current away from functional circuitry 110, ESD protector 140 protects functional circuitry 110 from undesirable exposure the ESD current. For example, ESD protector 140 may be connected, in parallel with functional circuitry 110, to terminal 120 of IC 100. ESD protector 140 may include input/output (I/O) ESD clamps 150a, 150b to clamp a high-voltage ESD to the high-voltage power terminal, or alternatively to clamp a low-voltage ESD to the low-voltage power terminal. Each of I/O ESD clamps 150a, 150b of ESD protector 140 may include a component that is adapted to transmit a signal that has a voltage greater than or less than a predetermined threshold value. For example, each of I/O ESD clamps 150a, 150b may include a diode or a field effect transistor (FET).
Meanwhile, the advancement of the design and manufacture of functional circuitry has resulted in functional circuitry with increased operating frequencies. For example, as the sizes of ICs have been scaled down, the operating frequencies of ICs have increased. However, ESD protector 140 commonly presents a parasitic capacitance to terminal 120 that undesirably filters the signal applied at terminal 120 as the signal passes to functional circuitry 110. For example, the diodes or FETs of I/O ESD clamps 150a, 150b may have parasitic capacitances. The parasitic capacitance of ESD protector 140 typically acts as a low-pass filter on the signal at terminal 120, producing an undesirable amount of high-frequency loss of the signal. In addition, ESD protectors that are more robust to larger ESD currents may result in more undesirable high-frequency loss than less robust ESD protectors. Thus, as the operating frequencies of the functional circuitry have increased, the parasitic capacitance of ESD protector 140 has become an increasingly significant problem.
A conventional ESD protector attempts to mitigate this problem by distributing ESD elements, such as diodes, along a transmission line between an input/output terminal and functional circuitry. Impedance components are arranged between the ESD elements to decrease the filtering effect of the ESD protector while still permitting clamping of a large current. However, this “distributed” ESD protector may consume an undesirably large amount of space. For example, the distributed ESD protector may consume space approximately in proportion to the number of distributed ESD elements. The distributed ESD protector may also not sufficiently decrease the filtering effect for certain implementations of functional circuitry.
In alternative conventional ESD protectors, an inductor is implemented in the ESD protector to lessen the filtering of the signal. The inductor includes a conductor arranged in a coil to generate a magnetic field when current is passed through the coil. However, the inductor may require specialized manufacturing processes and may also consume an undesirably large amount of space. The operation of the inductor may also cause undesirable magnetic or electric field disturbances in neighboring circuitry. These disturbances may become more acute as circuit size decreases or operating frequencies increase.